[Buildbot] #2945: Add an example of using codebases

[Buildbot trac]

#2945: Add an example of using codebases
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Reporter:  sa2ajj            |       Owner:  Ben
    Type:  enhancement       |      Status:  assigned
Priority:  patches-accepted  |   Milestone:  0.9.+
 Version:  master            |  Resolution:
Keywords:  docs              |
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Comment (by sa2ajj):

 One seemingly common use case: checkout two branches (of the same or
 different repositories) -- one according to the change, one according to
 the configuration.  With the "default" configuration, both are checked out
 with the revision in the change, which causes errors.

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Ticket URL: <http://trac.buildbot.net/ticket/2945#comment:5>
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